Theoretical Elucidation of Au(I)-Catalyzed Cycloisomerizations of Cycloalkyl-substituted 1,5-Enynes: 1,2-alkyl Shift versus C-H Bond Insertion Products

The Au(I)-catalyzed cycloisomerization reactions of cycloalkyl-substituted 1,5-enynes (A) have been investigated by performing density functional theory (DFT) calculations. Theoretical calculations suggest that the reaction proceeds via a stepwise mechanism by the initial formation of a Au(I)-carbene intermediate (B), followed by a 1,2-alkyl shift or C-H insertion reaction to form the ring-expanded three-cyclic product (C) or ring-closed four-cyclic product (D) depending on the size of cycloalkyl substitutions. It is found that the formation of intermediate B is the rate-determining step, and the formation of products C or D is controlled by the size (n) of cycloalkyl substitutions in 1,5-enynes. In the situations with n = 1 and 2, the calculated relative free energies and the barriers consistently indicate that the 1,5-enynes prefer to evolve into product C to product D. In contrast, for the situation of n = 4, the barrier forming product C is found to be higher than that forming product D, supporting the experimental observation that a range of the 1,5-enynes with n = 4 isomerize into product D, although it is thermodynamically less favorable than product C. The present theoretical results provide insight into the mechanism details of the catalytic rearrangement of 1,5-enynes and rationalize the early experimental observations well.